Wells for production of coal seam gas may be completed across multiple coal seams using open hole techniques or a completion structure, such as a slotted or perforated liner. Traditionally, for wells completed with a slotted or perforated liner, attribution of water or gas production to a particular coal seam may be difficult due to commingling of production from the multiple coal seams.
Conventionally, completed wells having slotted or perforated liners may include an annulus between the liner and the sidewall of the borehole. This annulus provides a pathway for fluids and gases to move from the coal seam longitudinally along the annulus as well as radially towards and through the slotted or perforated liner. The longitudinal and/or radial pathways may complicate determination of fluid and gas inflow rates from coal seams intersected by the borehole. These complications may include:
A) difficulty in attributing gas production to a particular coal seam. This difficulty may result in problems in determining gas reserves for particular coals seams, complicating determination of proven and probable reserves and therefore, Final Investment Decisions (FID). A FID is a final decision of the Capital Investment Decision (CID), as part of the long term corporate finance decisions based on key criteria to manage company's assets and capital structure. It normally follows on from project definition, which is associated with Front End Engineering and Design (FEED)
B) uncertainty regarding which coal seam or seams are producing gas and/or water during drawdown of water or the production lifecycle; this uncertainty may result in suboptimal selection of which seams to complete in other wells or to “squeeze” with cement in producing wells.
C) uncertainty regarding to what extent the slotted or perforated liner completion methods could improve gas production rate and reduce water production rates.